Method for indicating control channel in wireless access system, and base station and user equipment for the same

ABSTRACT

A method for transmitting/receiving data in a wireless access system, and a base station (BS) and a user equipment (UE) for the same are disclosed. The method for performing synchronization with a neighbor base station (BS) of a user equipment (UE) in a wireless access system includes, if a user equipment (UE) based on a macro BS is located at a boundary of the neighbor BS, measuring a synchronization channel of the neighbor BS, wherein a subframe of the neighbor BS includes an additional secondary synchronization signal (SSS) so as to perform synchronization with the neighbor BS.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2012/008271, filed on Oct. 11, 2012,which claims the benefit of U.S. Provisional Application No. 61/566,007,filed on Dec. 2, 2011, the contents of which are hereby incorporated byreference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless access system, and moreparticularly to a method for performing management in a heterogeneouscell using an Enhanced Physical Downlink Control Channel (ePDCCH), andan apparatus for supporting the same.

BACKGROUND ART

The most important one of requirements of a next-generation wirelessaccess system is to support a high data transfer rate. To achieve this,various technologies such as Multiple Input Multiple Output (MIMO),Cooperative Multiple Point Transmission (CoMP), relay, etc. have beendeveloped and studied.

Although downlink and uplink bandwidths are different from each other, aconventional wireless communication system typically uses one carrier.For example, a wireless communication system having one carrier for eachof downlink and uplink and symmetry between downlink and uplinkbandwidths may be provided based on a single carrier.

However, in order to guarantee a broadband bandwidth capable ofsatisfying a higher data transfer rate considering that frequencyresources are saturated, carrier aggregation (CA)/multiple cellstechnology has been proposed, which is designed for each bandwidth tosatisfy basic requirements capable of operating an independent systemand aggregates a plurality of bandwidths using a single system.

In this case, a bandwidth-based carrier capable of being independentlyoperated may be referred to as a component carrier (CC). In order tosupport the increasing transmission capacity, a bandwidth of the latest3GPP LTE-A or 802.16m has been continuously extended up to 20 MHz ormore. In this case, one or more component carriers (CCs) are aggregatedto support a broadband. For example, provided that one CC supports abandwidth of 5 MHz, 10 MHz or 20 MHz, a maximum of 5 CCs are aggregatedto support a system bandwidth of up to 100 MHz.

DISCLOSURE Technical Problem

Accordingly, the present invention is directed to a method forindicating a control channel in a wireless access system, and a basestation (BS) and a user equipment (UE) for the same that substantiallyobviate one or more problems due to limitations and disadvantages of therelated art. An object of the present invention is to provide a methodand apparatus for smoothly transmitting/receiving UL/DL data between abase station (BS) and a user equipment (UE) in a wireless access system(preferably, a wireless access system supporting carrier aggregation).

Another object of the present invention is to provide a method forreducing the influence of interference encountered either betweenhomogeneous networks or between heterogeneous networks, and a method andapparatus for operating/managing additional secondary synchronizationsignals (SSSs) used for reducing interference between a primarysynchronization signal (PSS) and a secondary synchronization signal(SSS) in a heterogeneous network.

It will be appreciated by persons skilled in the art that the objectsthat can be achieved through the present invention are not limited towhat has been particularly described hereinabove and the above and otherobjects that the present invention can achieve will be more clearlyunderstood from the following detailed description taken in conjunctionwith the accompanying drawings.

Technical Solution

The object of the present invention can be achieved by providing amethod for performing synchronization with a neighbor base station (BS)of a user equipment (UE) in a wireless access system, the methodincluding: if a user equipment (UE) based on a macro BS is located at aboundary of the neighbor BS, measuring a synchronization channel of theneighbor BS, wherein a subframe of the neighbor BS includes anadditional secondary synchronization signal (SSS) so as to performsynchronization with the neighbor BS.

In another aspect of the present invention, a user equipment (UE) foracquiring synchronization with a neighbor base station (BS) in awireless access system includes a processor configured to measure asynchronization channel of the neighbor BS, if a user equipment (UE)based on a macro BS is located at a boundary of the neighbor BS, whereina subframe of the neighbor BS includes an additional secondarysynchronization signal (SSS) so as to perform synchronization with theneighbor BS.

The additional SSS may be located in a subframe established as an almostblank subframe (ABS) in the macro BS.

At least one additional SSS may be transmitted within one subframe.

The additional SSS may be located in a subframe having no CRS (CommonReference Signal).

The additional SSS may be allocated to a fixed position, and includespecific information indicating the presence or absence of theadditional SSS.

A resource of the macro BS corresponding to a subframe for transmissionof the additional SSS may be muted.

Advantageous Effects

Exemplary embodiments of the present invention have the followingeffects. In accordance with an embodiment of the present invention,UL/DL data can be easily communicated between a user equipment (UE) anda base station (BS) in a wireless access system (preferably, a wirelesscommunication system capable of supporting carrier aggregation (CA)).

The embodiment of the present invention can transmit additional SSSsignals capable of obviating the interference problem encounteredbetween PSS and SSS in a heterogeneous network, so that it canefficiently obtain synchronization.

In accordance with the embodiment of the present invention, due tointerference reduction, the user equipment (UE) can improve a cellthroughput and difficulty of UE implementation can be greatly reduced.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved through the present invention are not limited towhat has been particularly described hereinabove and other advantages ofthe present invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 is a conceptual diagram illustrating physical channels used in a3GPP LTE system and a general method for transmitting a signal using thephysical channels.

FIG. 2 is a diagram illustrating a structure of a radio frame for use ina 3GPP LTE system.

FIG. 3 exemplarily shows a resource grid of a single downlink slot.

FIG. 4 illustrates an uplink subframe structure.

FIG. 5 illustrates a downlink frame structure.

FIG. 6 exemplarily shows a component carrier (CC) for use in an LTEsystem and carrier aggregation (CA) for use in an LTE A system.

FIG. 7 shows a subframe structure of the LTE-A system according tocross-carrier scheduling.

FIG. 8 is a diagram illustrating a heterogeneous network structure.

FIG. 9 is a flowchart illustrating a general handover procedure.

FIG. 10 exemplarily shows an event triggered reporting condition A3.

FIG. 11 exemplarily shows Cell Range Extension (CRE).

FIG. 12 exemplarily shows interference encountered between heterogeneousnetwork wireless communication systems including a macro cell and amicro cell.

FIG. 13 exemplarily shows an Almost Blank Subframe (ABS) for use in amacro cell under a macro-pico network.

FIG. 14 exemplarily shows a Closed Subscriber Group (CSG) scenarioserving as an example of a time-domain Inter-Cell InterferenceCoordination (ICIC).

FIG. 15 exemplarily shows a pico scenario serving as a time-domain ICIC.

FIG. 16 shows a radio FDD (Frequency Division Duplexing) frame structurefor use in the 3GPP LTE system under a normal CP (Cyclic Prefix).

FIG. 17 exemplarily shows an Almost Blank Subframe (ABS) based on theconcept of FIG. 13.

FIG. 18 shows two kinds of ABS.

FIG. 19 exemplarily shows a subframe shift and a symbol level shift.

FIG. 20 exemplarily shows an ABS, a subframe shift, and a symbol levelshift.

FIG. 21 exemplarily shows autocorrelation based on a common referencesignal (CRS) planted at intervals of three spaces in a frequency domain.

FIG. 22 exemplarily shows synchronization acquisition using additionalsignals.

FIG. 23 shows mutual interference environment of a synchronous channelin a heterogeneous network.

FIG. 24 is a block diagram illustrating a wireless communicationapparatus according to an embodiment of the present invention.

BEST MODE

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain exemplaryembodiments of the present invention, rather than to show the onlyembodiments that can be implemented according to the present invention.The following detailed description includes specific details in order toprovide a thorough understanding of the present invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced without such specific details.

In some cases, in order to prevent ambiguity of the concepts of thepresent invention, conventional devices or apparatuses well known tothose skilled in the art will be omitted and be denoted in the form of ablock diagram on the basis of important functions of the presentinvention.

The embodiments of the present invention are disclosed on the basis of adata communication relationship between a base station and a terminal.In this case, the base station is used as a terminal node of a networkvia which the base station can directly communicate with the terminal.Specific operations to be conducted by the base station in the presentinvention may also be conducted by an upper node of the base station asnecessary. In other words, it will be obvious to those skilled in theart that various operations for enabling the base station to communicatewith the terminal in a network composed of several network nodesincluding the base station will be conducted by the base station orother network nodes other than the base station. The term “Base Station(BS)” may be replaced with a fixed station, Node-B, eNode-B (eNB), or anaccess point (AP) as necessary. The term “relay” may be replaced with aRelay Node (RN) or a Relay Station (RS). The term “terminal” may also bereplaced with a User Equipment (UE), a Mobile Station (MS), a MobileSubscriber Station (MSS) or a Subscriber Station (SS) as necessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for the convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to another format within the technical scope orspirit of the present invention.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3^(rd) Generation Project Partnership (3GPP) system, a3GPP Long Term Evolution (LTE) system, an LTE-Advanced (LTE-A) system,and a 3GPP2 system. In particular, the steps or parts, which are notdescribed to clearly reveal the technical idea of the present invention,in the embodiments of the present invention may be supported by theabove documents. All terminology used herein may be supported by atleast one of the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, CDMA (CodeDivision Multiple Access), FDMA (Frequency Division Multiple Access),TDMA (Time Division Multiple Access), OFDMA (Orthogonal FrequencyDivision Multiple Access), SC-FDMA (Single Carrier Frequency DivisionMultiple Access), and the like. The CDMA may be embodied with wireless(or radio) technology such as UTRA (Universal Terrestrial Radio Access)or CDMA2000. The TDMA may be embodied with wireless (or radio)technology such as GSM (Global System for Mobile communications)/GPRS(General Packet Radio Service)/EDGE (Enhanced Data Rates for GSMEvolution). The OFDMA may be embodied with wireless (or radio)technology such as Institute of Electrical and Electronics Engineers(IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA(Evolved UTRA). The UTRA is a part of the UMTS (Universal MobileTelecommunications System). The 3GPP (3rd Generation PartnershipProject) LTE (long term evolution) is a part of the E-UMTS (EvolvedUMTS), which uses E-UTRA. The 3GPP LTE employs the OFDMA in downlink andemploys the SC-FDMA in uplink. The LTE-Advanced (LTE-A) is an evolvedversion of the 3GPP LTE.

For clarity, the following description focuses on the 3GPP LTE and 3GPPLTE-A system. However, technical features of the present invention arenot limited thereto.

1. Overview of 3GPP LTE/LTE-A Systems Applicable to the PresentInvention

1.1 Overview of System

FIG. 1 is a conceptual diagram illustrating physical channels for use ina 3GPP system and a general method for transmitting a signal using thephysical channels.

Referring to FIG. 1, when powered on or when entering a new cell, a UEperforms initial cell search in step S11. The initial cell searchinvolves synchronization with a BS. Specifically, the UE synchronizeswith the BS and acquires a cell Identifier (ID) and other information byreceiving a Primary Synchronization CHannel (P-SCH) and a SecondarySynchronization CHannel (S-SCH) from the BS.

Then the UE may acquire information broadcast in the cell by receiving aPhysical Broadcast CHannel (PBCH) from the BS. During the initial cellsearch, the UE may monitor a downlink channel status by receiving adownlink Reference Signal (DL RS).

After initial cell search, the UE may acquire more specific systeminformation by receiving a Physical Downlink Control CHannel (PDCCH) andreceiving a Physical Downlink Shared CHannel (PDSCH) based oninformation of the PDCCH in step S12.

Thereafter, if the UE initially accesses the BS, it may perform randomaccess to the BS in steps S13 to S16. For random access, the UE maytransmit a preamble to the BS on a Physical Random Access CHannel(PRACH) in step S13 and receive a response message for the random accesson a PDCCH and a PDSCH corresponding to the PDCCH in step S14. In thecase of contention-based random access, the UE may transmit anadditional PRACH in step S15, and receive a PDCCH and a PDSCHcorresponding to the PDCCH in step S16 in such a manner that the UE canperform a contention resolution procedure.

After the above random access procedure, the UE may receive aPDCCH/PDSCH (S17) and transmit a Physical Uplink Shared CHannel(PUSCH)/Physical Uplink Control CHannel (PUCCH) (S18) in a generaluplink/downlink signal transmission procedure.

Control information that the UE transmits to the BS is referred to asuplink control information (UCI). The UCI includes a Hybrid AutomaticRepeat and reQuest ACKnowledgment/Negative-ACK (HARQ ACK/NACK) signal, aScheduling Request (SR), Channel Quality Indictor (CQI), a PrecodingMatrix Index (PMI), and a Rank Indicator (RI).

In the LTE system, UCI is transmitted on a PUCCH, in general. However,the UCI can be transmitted on a PUSCH when control information andtraffic data need to be transmitted simultaneously. Furthermore, the UCIcan be aperiodically transmitted on a PUSCH at the request/instructionof a network.

FIG. 2 is a diagram illustrating a structure of a radio frame for use ina 3GPP LTE system.

FIG. 2(a) illustrates a frame structure type 1. The Type-1 framestructure shown in FIG. 2(a) may be applied to a Frequency DivisionDuplexing (FDD) system and a half-FDD (H-FDD) system.

Referring to FIG. 2(a), one radio frame has a length of 10 ms(T_(f)=327200·T_(s)=10 ms). The single radio frame is divided into 20equally-sized slots, each of which is 0.5 ms long(T_(slot)=15360·T_(s)=0.5 ms). The 20 slots may be sequentially numberedfrom 0 to 19. One subframe includes two contiguous slots. An i-thsubframe includes a slot (2i) and a slot (2i+1). That is, the radioframe includes subframes. A time required for transmitting one subframeis defined as a Transmission Time Interval (TTI). Here, T_(s) denotes asampling time, and is expressed by ‘T_(s)=1/(15 kHz×2048)=3.2552×10⁻⁸(about 33 ns)’. The slot includes a plurality of OFDM symbols in a timedomain, and includes a plurality of resource blocks (RBs) in a frequencydomain.

One slot includes a plurality of OFDM symbols in the time domain. Since3GPP LTE uses OFDMA in downlink, the OFDM symbol is used to indicate onesymbol period. The OFDM symbol may be referred to as one SC-FDMA symbolor one symbol period. The resource block is a resource allocation unit,and includes a plurality of contiguous subcarriers in one slot.

During each period of 10 ms for use in the full-duplex FDD system, 10subframes can be simultaneously used for UL and DL transmission. In thiscase, the UL transmission and the DL transmission are separated fromeach other in the frequency domain. In contrast, a user equipment (UE)for use in the half-duplex FDD system is unable to simultaneouslyperform transmission and reception operations.

The above-mentioned radio frame structure is only exemplary.Accordingly, the number of subframes included in the radio frame, thenumber of slots included in the subframe or the number of symbolsincluded in the slot may be changed in various manners.

FIG. 2(b) illustrates a frame structure type 2. The frame structure type2 is applied to a TDD system. One radio frame has a length of 10 ms(T_(f)=327200·T_(s)=10 ms), and is composed of two half-frames eachhaving a length of 5 ms (153600·T_(s)=5 ms). Each half-frame includes 5subframes each having a length of 1 ms (32720·T_(s)=1 ms). The i-thsubframe includes two slots (2i, 2i+1) each having a length of 0.5 ms(T_(slot)=15360·T_(s)=0.5 ms). Here, T_(s) denotes a sampling time, andis expressed by ‘T_(s)=1/(15 kHz×2048)=3.2552×10⁻⁸ (about 33 ns)’.

The frame structure type 2 includes a special subframe composed of threefields, i.e., a downlink pilot time slot (DwPTS), a guard period (GP),and an uplink pilot time slot (UpPTS). In this case, DwPTS is used toperform initial cell search, synchronization, or channel estimation.UpPTS is used to perform channel estimation of a base station and uplinktransmission synchronization of a user equipment (UE). The guardinterval (GP) is located between an uplink and a downlink so as toremove interference generated in the uplink due to multi-path delay of adownlink signal.

Table 1 shows configuration of the special frame. That is, Table 1 showsDwPTS/GP/UpPTS lengths.

TABLE 1 Normal cyclic prefix in downlink Extended cyclic prefix indownlink UpPTS UpPTS Normal Extended Normal Extended Special subframecyclic prefix cyclic prefix cyclic prefix cyclic prefix configurationDwPTS in uplink in uplink DwPTS in uplink in uplink 0  6592 · T_(s) 2192· T_(s) 2560 · T_(s)  7680 · T_(s) 2192 · T_(s) 2560 · T_(s) 1 19760 ·T_(s) 20480 · T_(s) 2 21952 · T_(s) 23040 · T_(s) 3 24144 · T_(s) 25600· T_(s) 4 26336 · T_(s)  7680 · T_(s) 4384 · T_(s) 5120 · T_(s) 5  6592· T_(s) 4384 · T_(s) 5120 · T_(s) 20480 · T_(s) 6 19760 · T_(s) 23040 ·T_(s) 7 21952 · T_(s) — — — 8 24144 · T_(s) — — —

In the frame structure type 2, the UL-DL configuration indicates how allthe subframes are allocated (or reserved) to DL and UL. Table 2 showsUL-DL configuration.

TABLE 2 Uplink-downlink Downlink-to-Uplink Subframe number configurationSwitch-point periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U U D S U U U1 5 ms D S U U D D S U U D 2 5 ms D S U D D D S U D D 3 10 ms  D S U U UD D D D D 4 10 ms  D S U U D D D D D D 5 10 ms  D S U D D D D D D D 6 5ms D S U U U D S U U D

Referring to Table 2, D denotes a subframe for DL transmission, Udenotes a subframe for UL transmission, and S denotes a special subframecomposed of three fields (i.e., DwPTS, GP, UpPTS). The UL-DLconfiguration can be classified into 7 UL-DL configurations. Thepositions and numbers of DL subframes, special subframes, and ULsubframes are different from each other per UL-DL configuration.

A time at which DL is changed to UL or a time at which UL is changed toDL is referred to as a switching point. Switch-point periodicity denotesa period in which switching between a UL subframe and a DL subframe isequally repeated. The switch-point periodicity supports each of 5 ms and10 ms. In case of the 5 ms DL-UL switch-point period, a special subframe(S) is present per half-frame. In case of 5 ms DL-UL switch-pointperiod, the special subframe (S) is present only in a first half-frame.

In all the configurations, each of the 0^(th) subframe, the 5^(th)subframe, and DwPTS are used for DL transmission only. UpPTS and asubframe immediately subsequent to the special subframe (S) are alwaysused for UL transmission.

The above-mentioned UL-DL configuration is considered to be systeminformation well known to all of the BS and the UE. The BS is configuredto transmit only an index of configuration information whenever theUL-DL configuration information is changed, so that the change of aUL-DL allocation state of a radio frame can be recognized by the UE. Inaddition, configuration information is a kind of DL control information,and can be transmitted through a Physical Downlink Control Channel(PDCCH) serving as a DL control channel in the same manner as in otherscheduling information. The configuration information serving asbroadcast information can be commonly transmitted to all UEs containedin the cell through a broadcast channel. In the TDD system, the numberof half frames contained in a radio frame, the number of subframescontained in a half frame, and a combination of DL and UL subframes isdisclosed only for illustrative purposes.

Meanwhile, HARQ ACK/NACK transmitted to the UE over a PHICH at an i-thsubframe in the FDD system is associated with a PUSCH transmitted at an(i−4)-th subframe by the UE.

On the other hand, a DL/UL subframe configuration of the TDD system isdifferent per a UL-DL configuration, so that a PUSCH and PHICHtransmission time is different per configuration and the PUSCH and PHICHtransmission time may be differently constructed according to an index(or number) of a subframe.

In the LTE system, UL/DL timing relationship among a PUSCH, a PDCCHpreceding the PUSCH, and a PHICH for transmission of DL HARQ ACK/NACKcorresponding to the PUSCH is predetermined.

FIG. 3 exemplarily shows a resource grid of a single downlink slot.

Referring to FIG. 3, one downlink slot includes a plurality of OFDMsymbols in a time domain. Although one downlink slot includes 7 OFDMsymbols and one resource block (RB) includes 12 subcarriers in afrequency domain, the scope or spirit of the present invention is notlimited thereto.

Each element on a resource grid may be defined as a resource element(RE). One RB includes 12×7 REs. The number (N^(DL)) of RBs contained ina downlink slot is dependent upon a downlink transmission bandwidth. Anuplink slot structure is identical to the downlink slot structure.

FIG. 4 illustrates an uplink subframe structure.

Referring to FIG. 4, a UL subframe is divided into a control region anda data region in the frequency domain. PUCCH carrying UL controlinformation is allocated to the control region. PUSCH carrying user datais allocated to the data region. In order to maintain a single carrierproperty, one UE does not simultaneously transmit a PUCCH signal and aPUSCH signal. A PUCCH for one UE is allocated in an RB pair in asubframe and RBs belonging to the RB pair occupy different subcarriersin each of two slots. Thus, the RB pair allocated to the PUCCH is‘frequency-hopped’ at a slot boundary.

FIG. 5 illustrates a downlink frame structure.

Referring to FIG. 5, a maximum of three OFDM symbols located in thefront of a first slot of the subframe are used as a control region towhich control channels are allocated, and the remaining OFDM symbols areused as a data region to which a Physical Downlink Shared Channel(PDSCH) channel is allocated. DL control channels for use in the 3GPPLTE system include a Physical Control Format Indicator CHannel (PCFICH),a Physical Downlink Control Channel (PDCCH), a Physical Hybrid-ARQIndicator CHannel (PHICH), and the like.

PCFICH transmitted through a first OFDM symbol of the subframe may carryinformation about the number of OFDM symbols (i.e., size of the controlregion) used for transmission of control channels within the subframe.PHICH serving as a response channel to an uplink may carry ACK(Acknowledgement)/NACK (Non-Acknowledgement) signals about a HybridAutomatic Repeat Request (HARQ). Control information transmitted over aPDCCH is referred to as Downlink Control Information (DCI). For example,DCI includes uplink resource allocation information (UL grant), downlinkresource allocation information (DL grant), or an uplink transmission(UL Tx) power control command for an arbitrary UE group, etc.

1.2. Physical Downlink Control Channel (PDCCH)

1.2.1. Overview of PDCCH

PDCCH may carry information about resource allocation and transmissionformat (DL grant) of a downlink shared channel (DL-SCH), resourceallocation information (UL grant) of an uplink shared channel (UL-SCH),paging information of a paging channel (PCH), resource allocationinformation about an upper layer control message such as a random accessresponse transmitted over a PDSCH, a set of transmission power controlcommands for each UE contained in an arbitrary UE group, and informationabout Voice over Internet Protocol (VoIP) activation, etc.

A plurality of PDCCHs may be transmitted within the control region, andthe UE may monitor the PDCCHs. Each PFCCH includes an aggregate of oneor more contiguous control channel elements (CCEs). The PDCCH composedof the aggregate of one or more contiguous CCEs may be transmittedthrough the control region after performing subblock interleaving. CCEis a logical allocation unit for providing a coding rate based on aRadio frequency (RF) channel status to the PDCCH. CCE may correspond toa plurality of resource element groups. PDCCH format and the number ofavailable PDCCHs may be determined according to the relationship betweenthe number of CCEs and the coding rate provided by CCEs.

1.2.2. PDCCH Structure

A plurality of multiplexed PDCCHs for a plurality of UEs can betransmitted within a control region: PDCCH may be composed of one ormore contiguous CCE aggregations. CCE is a predetermined unitcorresponding to 9 sets of an REG composed of 4 resource elements (REs).4 QPSK (Quadrature Phase Shift Keying) symbols are mapped to each REG.Resource elements (REs) occupied by a reference signal (RS) are notcontained in the REG. That is, a total number of REGs contained in anOFDM symbol may be changed according to the presence or absence of acell-specific reference signal (RS). Concept of an REG configured to map4 resource elements (REs) to one group can also be applied to another DLcontrol channel (e.g., PDFICH or PHICH). Provided that an REG notallocated to PCFICH or PHICH is denoted by N_(REG), the number of CCEsavailable to the system is denoted by N_(CCE)=└N_(REG)/9┘, andindividual CCEs are indexed from 0 to N_(CCE)−1.

In order to simplify a decoding process of the UE, a PDCCH formatincluding n CCEs may start from a specific CCE having the same index asa multiple of n. That is, if the CCE index is denoted by i, the PDCCHformat may start from a CCE configured to satisfy i mod n=0.

The base station (BS) may use {1, 2, 4, 8} CCEs to configure one PDCCHsignal. Here, {1, 2, 4, 8} are referred to as CCE aggregation levels.The number of CCEs used to transmit a specific PDCCH is determined bythe BS according to a channel state. For example, a PDCCH for a UEhaving a good DL channel state (where the UE may be located close to theBS) can be sufficiently covered by only one CCE. In contrast, if a UEhaving a poor channel state (where the UE may be located at a celledge), 8 CCEs may be needed for sufficient robustness. In addition, aPDCCH power level may be mapped to a channel state, and then adjustedaccording to the channel state.

Table 3 shows a PDCCH format, and four PDCCH formats are supportedaccording to CCE aggregation levels.

TABLE 3 PDCCH Number of Number of Number of format CCEs (n) REGs PDCCHbits 0 1 9 72 1 2 18 144 2 4 36 288 3 8 72 576

The reason why a different CCE aggregation level is assigned to each UEis that a format or MCS (Modulation and Coding Scheme) of PDCCH controlinformation is differently assigned to each UE. MCS level denotes a coderate and a modulation order for use in the data coding. Adaptive MCSlevel is used for link adaptation. Generally, a control channel fortransmitting control information may consider about 3 or 4 MCS levels.

Control information transmitted over a PDCCH is referred to as downlinkcontrol information (DCI). Configuration of information loaded on aPDCCH payload may be changed according to a DCI format. PDCCH payloaddenotes information bits. Table 4 shows DCI according to DCI format.

TABLE 4 DCI Format Description Format Resource grants for the PUSCHtransmissions (uplink) 0 Format Resource assignments for single codewordPDSCH trans- 1 missions (transmission modes 1, 2 and 7) Format Compactsignaling of resource assignments for single codeword 1A PDSCH (allmodes) Format Compact resource assignments for PDSCH using rank-1 closed1B loop precoding (mode 6) Format Very compact resource assignments forPDSCH (e.g. paging/ 1C broadcast system information) Format Compactresource assignments for PDSCH using multi-user 1D MIMO (mode 5) FormatResource assignments for PDSCH for closed-loop MIMO 2 operation (mode 4)Format Resource assignments for PDSCH for open-loop MIMO opera- 2A tion(mode 3) Format Power control commands for PUCCH and PUSCH with 2-bit/3/3A 1-bit power adjustment

Referring to Table 4, DCI formats include format 0 for PUSCH scheduling,format 1 for scheduling of one PDSCH codeword, format 1A for compactscheduling of one PDSCH codeword, format 1C for compact scheduling ofDL-SCH, format 2 for PDSCH scheduling in a closed-loop spatialmultiplexing mode, format 2A for PDSCH scheduling in an open loopspatial multiplexing mode, and formats 3 and 3A for transmission oftransmission power control (TPC) commands for uplink channels. DCIformat 1A can be used for PDSCH scheduling irrespective of thetransmission mode of a UE.

The length of PDCCH payload may be changed according to DCI format. Inaddition, the type and length of PDCCH payload may be changed accordingto the presence or absence of compact scheduling or according to atransmission mode established in the UE.

The transmission mode may be configured for the UE to receive downlinkdata through a PDSCH. For example, downlink data through a PDSCH mayinclude UE-scheduled data, paging, a random access response, orbroadcast information through a BCCH. Downlink data through a PDSCH isassociated with DCI format signaled through a PDCCH. The transmissionmode may be semi-statically established in the UE through higher layersignaling (e.g., RRC (Radio Resource Control) signaling). Thetransmission mode can be classified into single antenna transmission ormulti-antenna transmission. The UE can semi-statically establish atransmission mode through higher layer signaling. For example, themulti-antenna transmission may include transmit diversity, open-loop orclosed-loop spatial multiplexing, MU-MIMO (Multi-User Multiple InputMultiple Output) or beamforming, etc. The transmit diversity canincrease transmission reliability by transmitting the same data throughmultiple transmission antennas. The spatial multiplexing enablesmultiple transmission antennas to simultaneously transmit different dataso that it can transmit high-speed data without increasing a systembandwidth. Beamforming can increase a Signal to Interference plus NoiseRatio (SINR) on the basis of a weight varying with a channel statususing multiple antennas.

DCI format is dependent upon a. UE-configured transmission mode. In caseof a reference DCI format, the UE can monitor necessary informationaccording to the UE-configured transmission mode. The UE-configuredtransmission mode may be classified into the following seventransmission modes (1) to (7).

1) Single antenna port: Port 0

2) Transmit Diversity

3) Open-loop Spatial Multiplexing

4) Closed-loop Spatial Multiplexing

5) MU-MIMO

6) Closed-loop rank=1 precoding

7) Single antenna port: Port 5

1.2.3. PDCCH Transmission

A base station (BS) decides a PDCCH format according to DCI to be sentto the UE, and adds a Cyclic Redundancy Check (CRC) to controlinformation. The CRC is masked with an identifier (e.g., a Radio NetworkTemporary Identifier (RNTI)) according to a PDCCH owner or a purpose ofthe PDCCH. For example, provided that the PDCCH is provided for aspecific UE, an identifier of the corresponding UE (e.g., cell-RNTI(C-RNTI)) may be masked with the CRC. If PDCCH is provided for a pagingmessage, a paging identifier (e.g., paging-RNTI (P-RNTI)) may be maskedwith a CRC. If a PDCCH is provided for system information (e.g., systeminformation block (SIB)), system information RNTI (SI-RNTI) may bemasked with CRC. In order to indicate a random access response totransmission of a UE random access preamble, random access-RNTI(RA-RNTI) may be masked with CRC.

The BS generates coded data by performing channel coding on CRC-addedcontrol information. In this case, the BS can perform channel coding ata code rate depending on the MCS level. The BS performs rate matchingaccording to a CCE aggregation level allocated to a PDCCH format.Thereafter, the BS modulates the coded data and thus generatesmodulation symbols. In this case, the BS may use a modulation sequencedepending on the MCS level. The modulation symbols contained in onePDCCH may correspond to one of CCE aggregation levels 1, 2, 4 and 8.Thereafter, the BS maps the modulation symbols to a physical resourceelement (PRE) (i.e., CCE to RE mapping).

1. 2. 4. Blind Decoding

A plurality of PDCCHs can be transmitted within one subframe. That is, acontrol region of one subframe is composed of a plurality of CCEs havingindices 0˜N_(CCE,k)−1. In this case, N_(CCE,k) is a total number of CCEscontained in a control region of the k-th subframe. The UE monitors aplurality of PDCCHs for each subframe. In this case, the term“monitoring” means that the UE attempts to decode each PDCCH accordingto a monitored PDCCH format. In the control region allocated to thesubframe, the BS does not provide the UE with the corresponding PDCCHposition information. In order to receive a control channel transmittedfrom the BS, the UE is unable to recognize where its own PDCCH istransmitted at a certain CCE aggregation level or a DCI format, so thatthe UE searches for the PDCCH by monitoring an aggregate of PDCCHcandidates within the subframe. The above-mentioned operation isreferred to as Blind Decoding/Detection (BD). The blind decoding (BD)means that a UE ID is demasked with a CRC and investigates CRC errors,so that it can allow the UE to recognize whether the corresponding PDCCHis a control channel of the UE. In the active mode, the UE monitors aPDCCH of each subframe so as to receive data to be sent to the UE. Inthe DRX mode, the UE is awakened from the monitoring section of each DRXperiod, and monitors a PDCCH in the subframe corresponding to themonitoring section. The subframe in which PDCCH monitoring is performedis referred to as a non-DRX subframe.

The UE must perform blind decoding (BD) of all CCEs contained in thecontrol region of the non-DRX subframe so as to receive a PDCCH to besent to the UE. Since the UE does not recognize which PDCCH format willbe transmitted, it must decode all PDCCHs at all available CCEaggregation levels until PDCCH blind decoding (BD) is successfullyperformed in each non-DRX subframe. The LTE system has defined a SearchSpace (SS) concept to perform UE blind decoding. The SS means a set ofPDCCH candidates for monitoring. The SS may have a different sizeaccording to each PDCCH format. The SS may be comprised of a CommonSearch Space (CSS) and a UE-specific/Dedicated Search Space (USS). Incase of the CSS, all UEs can recognize the size of CSS, but the USS maybe independently established for each UE. Therefore, the UE must monitorthe USS and the CSS to decode a PDCCH, so that the UE performs blinddecoding (BD) a maximum of 44 times within one subframe. In this case,blind decoding (BD) to be performed according to different CRC values(for example, C-RNTI, P-RNTI, SI-RNTI, RA-RNTI) is not performed.

Due to a small-sized search space, the BS may not acquire CCE resourcesused for transmitting a PDCCH to all UEs configured to transmit thePDCCH within a given subframe, because the remaining resources generatedafter the CCE position has been allocated may not be contained in asearch space (SS) of a specific UE. In order to minimize such barriercapable of being continued even in the next subframe, a UE specifichopping sequence may be applied to a start point of the UE-specific SS.

Table 5 shows the sizes of common search space (CSS) and UE-specificsearch space (USS).

TABLE 5 Number of Number of PDCCH Number of candidates in candidates informat CCEs (n) common search space dedicated search space 0 1 — 6 1 2 —6 2 4 4 2 3 8 2 2

In order to reduce UE calculation load dependent upon the number ofblind decoding (BD) attempt times, the UE does not simultaneouslyperform search actions for all the defined DCI formats. In more detail,the UE always searches for DCI format 0 and DCI format 1A in theUE-specific SS. In this case, although DCI format 0 and DCI format LAhave the same size, the UE can discriminate a DCI format using a “flagfor format 0/format 1A differentiation” contained in a PDCCH. Inaddition, other DCI formats may be requested except for DCI formats 0and 1A. For example, DCI format 1, DCI format 1B, and DCI format 2 maybe used.

In the common search space (CSS), the UE can search for DCI format 1Aand DCI format 1C. In addition, the UE may be configured to search forDCI format 3 or 3A. Although DCI formats 3 and 3A may have the same sizeas DCI formats 0 and 1A, the UE can discriminate a DCI format using aCRC scrambled by another ID instead of a UE-specific ID.

The search space S_(k) ^((L)) means a PDCCH candidate set according toan aggregation level Lε{1, 2, 4, 8}. CCE dependent upon a PDCCHcandidate set m of the search space can be determined by the followingequation 1.L·{(Y _(k) +m)mod └N _(CCE,k) /L┘}+i  [Equation 1]

In Equation 1, M^((L)) is the number of PDCCH candidates according to aCCE aggregation level L required for the monitoring action in the searchspace, where m is denoted by m=0, . . . , M^((L))−1. i is an index fordesignating each CCE at each PDCCH candidate a PDCCH, as represented byi=0, . . . , L−1. In Equation 1, k is denoted by k=└n_(s)/2┘, n_(s) is aslot index within a radio frame.

As described above, the UE monitors both a UE-specific search space(USS) and a common search space (CSS) to decode a PDCCH. Here, the CSSsupports PDCCHs having aggregation levels {4, 8}, and the USS supportsPDCCHs having aggregation levels {1, 2, 4, 8}. Table 6 shows PDCCHcandidates monitored by the UE.

TABLE 6 Search space S_(k) ^((L)) Number of PDCCH Type Aggregation levelL Size [in CCEs] candidates M^((L)) UE- 1 6 6 specific 2 12 6 4 8 2 8 162 Common 4 16 4 8 16 2

Referring to Equation 1, in case of the CSS, Y_(k) for two aggregationlevels L=4 and L=8 is set to zero ‘0’. In contrast, in case of the USS,Y_(k) for the aggregation level L is denoted by the following equation2.Y _(k)=(A·Y _(k-1))mod D  [Equation 2]

In Equation 2, Y_(k-1) is denoted by Y_(k-1)=n_(RNTI)≠0, n_(RNTI) is anRNTI value, A is set to 39827 (A=39827), and D is set to 65537(D=65537).

2. Carrier Aggregation (CA) Environment

2.1. General Carrier Aggregation (CA)

Environments that are taken into consideration in the embodiments of thepresent invention include all general multicarrier resourceenvironments. That is, the term “multicarrier system” or “carrieraggregation system” used in the present invention refers to a systemthat uses an aggregation of one or more carriers having a smallerbandwidth than a target bandwidth when configuring a target wideband inorder to support wideband.

In the present invention, the term “multicarrier” or “multiple carriers”refers to a carrier aggregation (or carrier linkage or combination).Here, the term “carrier aggregation” refers not only to an aggregate ofcontiguous carriers but also to an aggregate of non-contiguous carriers.Different numbers of component carriers may be aggregated for thedownlink and the uplink. Aggregation of the same number of DL CCs and ULCCs is referred to as symmetric aggregation, whereas aggregation ofdifferent numbers of DL CCs and UL CCs is referred to as asymmetricaggregation. The term “carrier aggregation” may be used interchangeablywith the terms “bandwidth aggregation” and “spectrum aggregation”.

Carrier aggregation (CA) that is constructed by combining two or morecomponent carriers (CCs) aims to support a bandwidth of up to 100 MHz inthe LTE-A system, When one or more carriers having a smaller bandwidththan the target bandwidth are combined (or aggregated), bandwidths ofthe carriers to be combined may be limited to bandwidths that are usedin the conventional IMT system in order to maintain backwardcompatibility with the conventional IMT system. For example, theconventional 3GPP LTE system supports bandwidths of 1.4, 3, 5, 10, 15,and 20 MHz and the LTE-advanced (LTE_A) system can support a greaterbandwidth than 20 MHz using only the bandwidths supported by the LTEsystem. In addition, the carrier aggregation (CA) system used in thepresent invention can also define a new bandwidth, regardless of thebandwidths used in the conventional system, to support carriercombination (i.e., carrier aggregation).

The LTE-A system uses the concept of a cell so as to manage radioresources. The above-mentioned carrier aggregation (CA) environment maybe referred to as a “multi-cell environment”. The cell is defined as acombination of a DL resource and a UL resource. Here, the UL resourcesare not an essential part. Accordingly, the cell can be configured usingDL resources only, or DL resources and UL resources. Provided that aspecific UE has only one configured serving cell, the UE may use one DLCC and one UL CC. However, if the specific UE has two or more configuredserving cells, it may have as many DL CCs as the number of cells and asmany UL CCs as the number of DL CCs or less. Alternatively, the specificUE may also have as many UL CCs as the number of cells and as many DLCCs as the number of UL CCs or less. That is, if the specific UE has aplurality of configured serving cells, it is possible to support thecarrier aggregation (CA) environment in which the number of DL CCs isless than the number of UL CCs. That is, the carrier aggregation (CA)may be understood as an aggregate of two or more cells having differentcarrier frequencies (i.e., different intermediate frequencies of thecell). In this case, the term “cell” must be distinguished from “cell”serving as a specific region covered by a general BS.

The cell for use in the LTE-A system includes a primary cell (PCell) anda secondary cell (SCell). Each of PCell and SCell may be used as aserving cell. For a UE that does not support carrier aggregation (CA)while in an RRC_connected state, only one serving cell configured with aPCell exists. Conversely, for a UE that is in an RRC_Connected state andsupports carrier aggregation (CA), one or more serving cells including aPCell and at least one SCell are contained in the entire serving cell.

The serving cells (PCell and SCell) may be configured through RRCparameters. PhysCellId is an identifier (ID) for identifying a physicallayer of the cell, and may be set to any one of integers from 0 to 503.SCellIndex is a short identifier (ID) used for identifying the SCell,and may be set to any one of integers from 1 to 7. ServCellIndex is ashort ID for identifying the serving cell (PCell or SCell), and may beset to any one of integers from 0 to 7. The integer ‘0’ is applied toPCell, and SCellIndex is preassigned to be applied to SCell. That is, acell having the smallest cell ID (or the lowest cell index) from amongServCellIndex is used as a PCell.

PCell is a cell that operates on a primary frequency (or a primary CC).PCell may be used to perform initial connection configuration orconnection reconfiguration, and may refer to a cell designated in ahandover process. In addition, PCell refers to a cell used as a centerof control-related communication from among the serving cells configuredin the carrier aggregation (CA) environment. That is, the UE can receiveand transmit a PDCCH within its own PCell. The UE may use only PCellwhen acquiring system information or changing the monitoring procedure.Evolved Universal Terrestrial Radio Access (E-UTRAN) may change only thePCell so as to perform handover of a UE supporting the CA environmentusing RRC connection reconfiguration (RRCConnectionReconfiguration)message of a higher layer including mobility control information(mobilityControlInfo).

SCell may refer to a cell operated in a secondary frequency (or asecondary CC). One PCell is allocated to a specific UE, and one or moreSCells may be allocated to the specific UE. SCell may be configuredafter RRC connection has been achieved, and may also be used to provideadditional radio resources. The remaining cells (i.e., SCell) other thanPCell from among serving cells configured in the carrier aggregation(CA) environment have no PUCCH. When SCell is added to the UE supportingthe CA environment, E-UTRAN can provide all system information relatedto the cell staying in an RRC_CONNECTED state through a specificdedicated signal. The change of system information may be controlled byrelease or addition of the related SCell. In this case, an RRCconnection reconfiguration (RRCConnectionReconfiguration) message of thehigher layer may be used. E-UTRAN can perform dedicated signaling havinga different parameter for each UE without broadcasting informationwithin the related SCell.

After an initial security activation procedure is started, an E-UTRANmay configure a network including one or more SCells in addition to aninitially configured PCell during a connection configuration procedure.In a multicarrier environment, each of a PCell and an SCell may serve asa CC. In the following embodiments, a Primary CC (PCC) may have the samemeaning as a PCell and a Secondary CC (SCC) may have the same meaning asan SCell.

FIG. 6 exemplarily shows a component carrier (CC) for use in an LTEsystem and carrier aggregation (CA) for use in an LTE_A system.

FIG. 6(a) shows a single carrier structure for use in the LTE system.CCs can be classified into DL CC and UL CC. One CC may have a frequencyrange of 20 MHz.

FIG. 6(b) shows a carrier aggregation (CA) structure for use in theLTE_A system. FIG. 6(b) shows an exemplary case in which 3 CCs eachhaving the frequency size of 20 MHz are combined. Although three DL CCsand three UL CCs exist, it should be noted that the number of DL CCs andthe number of UL CCs are not limited thereto. In case of carrieraggregation (CA), the UE can simultaneously monitor three CCs, canreceive DL signal and DL data, and can transmit UL signal and UL data.

If N DL CCs are managed in a specific cell, the network can allocate MDL CCs (where M≦N) to the UE. In this case, the UE monitors only Mlimited DL CCs and then receives DL signals. In addition, the networkhas priority over L DL CCs (where L≦M≦N) so that a main DL CC can beallocated to the UE. In this case, the UE must monitor L DL CCs. Theabove-mentioned scheme can also be applied to UL transmission.

Linkage between a carrier frequency (or DL CC) of DL resources and acarrier frequency (or UL CC) of UL resources may be indicated by eithera higher-layer message (such as an RRC message) or system information.For example, a combination of a DL resource and a UL resource can beconfigured by linkage defined by System Information Block Type2 (SIB2).In more detail, linkage may indicate the mapping relationship between aDL CC and a UL CC. The DL CC is used for transmission of a PDCCHcarrying the UL grant, and the UL CC is used for the UL grant. Inaddition, the linkage may indicate the mapping relationship between a DLCC (or UL CC) for transmission of HARQ data and a UL CC (or DL CC) fortransmission of HARQ ACK/NACK signal.

2.2. Cross Carrier Scheduling

From the viewpoint of scheduling in a carrier (or carrier) or a servingcell for use in the carrier aggregation (CA) system, two schedulingmethods, i.e., a self-scheduling method and a cross carrier Schedulingmethod, may be used. The cross carrier scheduling may be referred to ascross component carrier scheduling or cross cell scheduling.

In case of the self-scheduling, PDCCH (DL grant) or PDSCH may betransmitted to the same DL CC, or PUSCH that is transmitted according toa PDCCH (UL grant) transmitted at DL CC is transmitted through a UL CClinked to a DL CC having received the UL grant.

In case of the cross carrier scheduling, PDCCH (DL grant) and PDSCH aretransmitted to different DL CCs, and a PUSCH that is transmittedaccording to a PDCCH (UL grant) transmitted at DL CC is transmittedthrough another UL CC but not a UL CC linked to a DL CC having receivedthe UL grant.

Execution or non-execution of cross carrier scheduling may beUE-specifically activated or deactivated. Through higher layer signaling(e.g., RRC signaling), information about the activation or deactivationcan be notified to each UE.

If cross carrier scheduling is activated, a carrier indicator field(CIF) is needed for a PDCCH. The CIF indicates which DL/UL CC is usedfor transmission of PDSCH/PUSCH indicated by the corresponding PDCCH.For example, PDCCH can allocate a PDSCH resource or a PUSCH resource toone of CCs using the CIF. That is, in the case where the PDSCH or PUSCHresource is allocated to one of DL/UL CCs in which a PDCCH on DL CC isaggregated in a multiplex manner, the CIF is configured. In this case,the DCI format of LTE-A Release-8 can be extended according to a CIF. Inthis case, the CIF may be fixed to a 3-bit field, or the location of theconfigured CIF may be fixed irrespective of the DCI format size. Inaddition, a PDCCH structure (i.e., resource mapping based on the samecoding and the same CCE) of LTE-A Release-8 may be reused.

In contrast, in the case where a PDCCH on a DL CC allocates a PDSCH onthe same DL CC or allocates a PUSCH resource on a single linked UL CC,no CIF is configured. In this case, the same PDCCH structure and DCIformat (resource mapping based on the same coding and the same CCE) asthose of LTE-A Release-8 may be used.

If it is possible to perform cross carrier scheduling, there is a needfor the UE to monitor a PDCCH of multiple DCIs in the control region ofthe monitoring CC according to a per-CC transmission mode and/or per-CCbandwidth. Therefore, not only configuration of a search space capableof supporting the above-mentioned operation but also PDCCH monitoring isneeded.

In the carrier aggregation (CA) system, a UE DL CC set may represent aset of DL CCs scheduled for the UE to receive a PDSCH, and a UE UL CCset may represent a set of UL CCs scheduled for the UE to transmit aPUSCH. In addition, the term “PDCCH monitoring set” indicates a set ofone or more DL CCs configured to perform PDCCH monitoring. The PDCCHmonitoring set may be identical to the UE DL CC set, or may be identicalto a subset of UE DL CC sets. The PDCCH monitoring set may include atleast one of DL CCs contained in the UE DL CC set. Alternatively, thePDCCH monitoring set may be separately defined irrespective of the UE DLCC set. DL CC contained in the PDCCH monitoring set may be configured ina manner that self-scheduling of the linked UL CC can always beexecuted. The UE DL CC set, the UE UL CC set, and the PDCCH monitoringset may be UE-specifically, UE group-specifically, or cell-specificallyconfigured.

If cross carrier scheduling is deactivated, this means that the PDCCHmonitoring set is always identical to the UE DL CC set. In this case, itis not necessary to perform indication such as separate signaling aboutthe PDCCH monitoring set. However, if cross carrier scheduling isactivated, it is preferable that the PDCCH monitoring set be defined inthe UE DL CC set. In other words, in order to schedule a PDSCH or PUSCHfor the UE, the base station (BS) transmits a PDCCH only through thePDCCH monitoring set.

FIG. 7 shows a subframe structure of the LTE-A system according tocross-carrier scheduling.

Referring to FIG. 7, a DL subframe for an LTE-A UE is combined withthree DL CCs. DL CC ‘A’ indicates that a PDCCH monitoring DL CC isconfigured. If a CIF is not used, each DL CC can transmit a PDCCHconfigured to schedule its own PDSCH without using the CIF. In contrast,if the CIF is used through higher layer signaling, only one DL CC ‘A’can transmit a PDCCH that is used for scheduling either PDSCH of the DLCC ‘A’ or a PDSCH of another CC. In this case, DL CC ‘B’ and DL CC ‘C’not established as PDCCH monitoring DL CCs do not transmit a PDCCH.

3. Heterogeneous Network Structure

FIG. 8 is a diagram illustrating a heterogeneous network structure.Referring to FIG. 8, in order to more stably guarantee a data servicesuch as a multimedia in the next-generation mobile communication, manydevelopers and companies are conducting intensive research into either ahierarchical cell structure in which micro cells (i.e., a pico celland/or a femto cell) for implementing low-power/short-distancecommunication are mixed in a macro-cell based homogeneous network, or aheterogeneous cell structure. Additional installation of a macro eNode-Bis far from efficient in terms of costs and complexity needed for highersystem performance. The heterogeneous network structure being consideredin the current communication network may be configured and achieved asshown in FIG. 10.

4. General Handover Procedure

A basic handover procedure is classified into the following four steps(1) to (4) shown in FIG. 9.

(1) Measurement Reporting:

If a handover trigger occurs when a UE monitors a measurement valuereceived from a serving cell or a neighbor cell, the UE reports thecorresponding measurement value to a serving UE.

(2) Handover (HO) Request:

The corresponding serving eNB having received the handover measurementreport from the UE transmits a handover (HO) request message to a targeteNB for HO.

(3) Handover (HO) Request Confirmation:

The eNode B having received the HO request transmits a HO confirmationmessage to the corresponding eNode B.

(4) Handover (HO) Command:

The serving eNB having received the HO confirmation message transmits aHO command to the corresponding UE so that the UE can be handed over tothe target eNB.

5. Measurement Reporting

In case of reporting configuration, the corresponding UE can report ameasurement value to the corresponding eNB using the following eventtriggered reporting criteria.

(1) Event A1: A measurement value of the serving cell is better than anabsolute threshold value.

(2) Event A2: A measurement value of the serving cell is worse than anabsolute threshold value.

(3) Event A3: A measurement value is better than an offset related to aneighbor-cell serving cell.

(4) Event A4: A measurement value of the neighbor cell is better than anabsolute threshold value.

(5) Event A5: A measurement value of the serving cell is worse than anabsolute threshold value, and a measurement value of the neighbor cellis better than another absolute threshold value.

FIG. 10 exemplarily shows an event triggered reporting condition A3.That is, FIG. 10 shows the event triggered reporting conditioncorresponding to Event A3. Provided that an RSRP value from the servingcell is greater than an RSRP from a neighbor cell by a specific offsetor more, if the corresponding condition is continuously satisfied afterlapse of a constant time-to-trigger time, triggering is executed.

6. Cell Range Extension: CRE)

FIG. 11 exemplarily shows Cell Range Extension (CRE).

Cell Range Extension (CRE) indicates that UEs, that are located in thevicinity of a pico eNB (e.g., PeNB) and receive interference from thePeNB, from among MUEs connected to a macro eNB (e.g., MeNB) under theheterogeneous network environment are handed over to the PeNB. Influenceof legacy interference can be reduced through such CRE execution, andload balancing can be achieved. If the serving cell is determinedthrough single measurement comparison such as legacy RSRP, there is ahigh possibility that an RSRP value from the macro eNB (MeNB) havinghigh transmission power is relatively higher than an RSRP from a PeNBhaving low power. Therefore, it is difficult for the corresponding MUEto be handed over to the corresponding PeNB. Therefore, CRE can beexecuted using the following criterion so as to implement smoothexecution of the CRE technology.Pico RSRP+offset>Macro RSRP  [Equation 3]

In Equation 3, an offset value is configured by higher layer signaling,and CRE to a PeNB having low transmission power can be executed.

Therefore, if each of a pico RSRP and an offset value is higher than amacro RSRP, CRE to a PeNB is executed.

7. Allocation of Almost Blank Subframe (ABS)

Heterogeneous network/deployments refers to a specific network structurein which micro cells for low-power/short-distance communication in amacro-cell based homogeneous network are mixed. The macro cell haslarger coverage and higher transmission power, and refers to a generalcell (or eNB) of a wireless communication system. The micro cell (ormicro eNB) is a small-sized version of the macro cell, such that themicro cell may independently perform most of the functions of the macrocell. The micro cell may be installed (in an overlay manner) in an areacovered by the macro cell or may be installed (in a non-overlay manner)in a shadow area that cannot be covered by the macro cell. The microcell has a narrower coverage and lower transmission power and mayaccommodate a smaller number of user equipments (UEs), compared to themacro cell. The micro cell may be referred to as a pico cell, a femtocell, a Hom evolved Node B (HeNB), a relay, etc.

UE may be directly served by the macro cell or the micro cell. In somecases, the UE present within the coverage of the micro cell may beserved by the macro cell.

The micro cell may be classified into two types according to accesslimitations of the UE. The first type is a Closed Subscriber Group (CSG)cell that prevents access of either a legacy macro UE (i.e., a UE servedby the macro cell) or another micro UE (i.e., a UE served by the microcell) without authentication. The second type is an Open AccessSubscriber Group (OASC) or Open Subscriber Group (OSC) cell that allowsaccess of the legacy macro UE or another micro UE.

Under the heterogeneous network environment in which the macro cell andthe micro cell coexist, more serious inter-cell interference than thehomogeneous network environment including only the macro cell (or themicro cell) may occur.

FIG. 12 exemplarily shows interference encountered between heterogeneousnetwork wireless communication systems including a macro cell and amicro cell.

Referring to the case (a) of FIG. 12, the macro UE (MUE) in which accessof the CSG cell is not permitted is interfered by HeNB. Referring to thecase (b) of FIG. 12, the macro UE generates serious interference towardthe HeNB. Referring to the case (c) of FIG. 12, the CSG UE is interferedby another CSG cell. Referring to the case (d) of FIG. 12, althoughuplink can be improved due to the use of cell-associated based passloss(e.g., a deviated Reference Signal Received Power (RSRP)), downlinkinterference of a UE but not the macro UE may increase.

The above-mentioned cases mean that not only UL and DL interferenceaffecting data, but also L1 (1^(st) layer)/L2 (2^(nd) layer) controlsignaling and methods for handling a synchronization signal and areference signal are of importance. The above-mentioned methods mayoperate in a time domain, a frequency domain, and/or a spatial region.

A macro-pico heterogeneous network and a macro cell may generate stronginterference in a UE served by a pico cell (specially, a UE located at aboundary of the serving pico-cell). For time-domain Inter-CellInterference Coordination (ICIC), the macro cell configured to generateinterference provides a subframe called an ABS (or ABSF: Almost BlankSubframe), so that the ABS or ABSF does not transmit a certain DLcontrol channel and data channel other than a CRS, so that the ABS orABSF can be protected from strong interference caused by the macro cell.Provided that Primary Synchronization Sequence (PSS), SecondarySynchronization Sequence (SSS), Physical Broadcast Control Channel(PBCH), System Information Block Type 1 (SIB1), Paging, PositioningReference Signal (PRS) are identical to the above-mentioned ABS, theabove-mentioned information is transmitted through the ABS. In addition,if the ABS is identical to a Multicast broadcast single frequencynetwork (MBSFN) subframe configured to transmit no signal in the dataregion, CRS is not transmitted in the data region of the ABS.

FIG. 13 exemplarily shows an Almost Blank Subframe (ABS) for use in amacro cell under a macro-pico network.

Referring to FIG. 13, the macro cell allows each of subframes #2 and #6to be composed of an ABSF, and such information may be indicated for apico cell through a backhaul. The pico cell can schedule a pico UE (thatis served by a pico cell) on the basis of information received from themacro cell. Specifically, the pico cell may schedule a plurality of UEslocated at a boundary of the macro cell and the pico cell within theABSF only. That is, the pico UE may perform CSI measurement only inABSFs.

In order to prevent an unnecessary radio link failure (RLF) as well asto correctly measure a Reference Signal Received Power (RSRP), aReference Signal Received Quality (RSRQ), etc., the interfered UE isconfigured to perform Radio Link Monitoring (RLM)/Radio ResourceManagement (RRM) at subframe(s) limited by the serving cell. For thispurpose, although bitmap signaling (for example, 1 denotes ABS and 0denotes the remaining frames) having the same period as that of backhaulsignaling can be used, a pattern must be configured independently fromthe backhaul bitmap pattern.

The legacy ICIC technologies have difficulty in overcoming the sameco-channel interference, so that two scenarios (i.e., a CSG scenario anda pico scenario) are proposed. The CSG scenario and the pico scenariorelate to exemplary network structures for describing the basic conceptof a time-domain ICIC. Of course, the CSG scenario and the pico scenariocan also be applied to other network deployment scenarios.

FIG. 14 exemplarily shows a Closed Subscriber Group (CSG) scenarioserving as an example of a time-domain Inter-Cell InterferenceCoordination (ICIC).

Referring to FIG. 14, if a UE (hereinafter referred to as a non-memberUE) configured to prevent access to the CSG cell approaches a CSG cell,the principal interference state may occur. Due to network deploymentand strategy, it may be impossible for the UE affected by inter-cellinterference to be diverted to another Evolved Universal TerrestrialRadio Access (E-UTRA) carrier or another Radio Access Technology (RAT)carrier. The time-domain ICIC may be utilized for the non-member UE tobe served by the macro cell in the same frequency layer.

If ABSF is used for the CSG cell that can protect the subframe of thecorresponding macro cell from interference, the interference can bereduced. The non-member UE may be signaled in a manner that protectedresources for RRM, RLM and CSI measurement for the serving macro cellcan be used. It may be possible for the non-member UE to continuouslyreceive a service from the macro cell under strong interference from theCSG cell.

In RRC_CONNECTED, the network can recognize that the non-member UE isassociated with strong interference from the CSG cell throughmeasurement events defined in LTE Release-8/9, and RRM/RLM/CSImeasurement resources may be limited for the UE. In addition, thenetwork may limit RRM measurement resources for a neighbor cell so as tofacilitate mobility from the serving macro cell. The network may releaseRRM/RLM/CSI measurement resources when the UE does not receive seriousinterference from the CSG cell any longer.

FIG. 15 exemplarily shows a pico scenario serving as a time-domain ICIC.

Referring to FIG. 15, the time-domain ICIC may be utilized for the picaUE located at a boundary of the serving pico cell (e.g., a UE beingtraffic-off-loaded from a macro cell to a pico cell). The time-domainICIC (Inter-Cell Interference Coordination) may be utilized for theabove-mentioned UE to receive a service from the pico cell in the samefrequency layer. Such interference may be reduced because the macro celluses the ABSF to protect a subframe of the corresponding pico cell frominterference. The pico UE (served by the pico cell) may utilizeresources protected for RRM, RLM, and CSI measurement for the servingpico cell. For the pico UE, limitation of RRM/RLM/CSI measurementresources can implement more correct measurement of the pico cell understrong interference from the macro cell. The pico cell may selectivelyconfigure limited RRM/RLM/CSI measurement resources for only UEs relatedto strong interference from the macro cell. In addition, for the UEserved by the macro cell, the network may configure RRM measurementresource limitation for a neighbor cell so as to facilitate mobilityfrom the macro cell to the pico cell.

The scheme for transferring subframe pattern (e.g., ABS pattern)information between cells will hereinafter be described in detail.

A cell causing interference may inform another cell receiving theinterference of 2 bitmaps through an X2 interface. Each bitmap may havethe size of 40 bits, and may represent attributes of each subframe inunits of 40 subframes. A first bitmap indicates a subframe including theABS. That is, the first bitmap may correspond to a specific bitmap inwhich an ABS is denoted by ‘1’ and the remaining subframes are denotedby ‘0’. The second bitmap may correspond to a bitmap from among thefirst bitmaps. The second bitmap indicates a subframe to be establishedas the ABS at a very high probability. That is, a subframe certainlyestablished as the ABS in the second bitmap may correspond to a subsetof the subframe established as an ABS in the first bitmap. Such subsetmay be used by a receiver so as to configure limited RLM/RRMmeasurement. The serving cell may indicate actual resources for RLM/RRMand CSI through RRC signaling.

In order to indicate the ABS pattern from the macro cell to the picocell, a bitmap pattern is used. The period of a bitmap pattern is 40 mslong in the FDD system. The period of a bitmap pattern is 20 ms long incase of UL-DL configurations 1˜5 in the TDD system. In case of the UL-DLconfiguration 0, the period of a bitmap pattern is 70 ms long. In caseof the UL-DL configuration 6, the period of a bitmap pattern is 60 mslong.

The bitmap pattern may be semi-statically updated. In this case, updatetrigger may aperiodically occur or may also occur on the basis of anevent.

8. PBCH, PSS, SSS Structure

FIG. 16 shows a radio FDD (Frequency Division Duplexing) frame structurefor use in the 3GPP LTE system under a normal CP (Cyclic Prefix).

Referring to FIG. 16, the FDD frame is comprised of a total of 10subframes from the 0^(th) subframe to the 9th subframe. In addition, incase of a normal CP, each subframe is comprised of a total of 14 OFDMsymbols.

In the FDD frame structure, the 0^(th) and 5^(th) subframe (subframe #0and subframe #5 in FIG. 16) are configured to transmit a primarysynchronization channel (PSCH) and a secondary synchronization channel(SSCH) for synchronization signals. In addition, the 0^(th) subframe maybe configured to transmit not only the synchronization signal but also aphysical broadcast channel (PBCH). Therefore, the 0^(th) subframe foruse in the system may be configured to transmit SSCH, PSCH, and PBCH,and the 5^(th) subframe may be configured to transmit the SSCH and PSCH.Specifically, the 5^(th) OFDM symbol is an OFDM symbol for SSCHtransmission at each of the 0^(th) and 5^(th) subframes, the 6^(th) OFDMsymbol is an OFDM symbol for PSCH transmission, and each of the 7^(th)to 10^(th) OFDM symbols is an OFDM symbol for PBCH transmission at the0^(th) subframe.

Meanwhile, in case of a normal CP, if each subframe is an extended CP ina radio FDD frame structure of the 3GPP LTE system, each subframe iscomprised of a total of 12 OFDM symbols. In the FDD frame structure, the0^(th) and 5^(th) subframes are configured to transmit a primarysynchronization channel (PSCH) and a secondary synchronization channel(SSCH). Primary Synchronization Signal (PSS) may be used to acquiretime-domain synchronization and/or frequency-domain synchronization suchas OFDM symbol synchronization, slot synchronization, etc. SecondarySynchronization Signal (SSS) may be used to acquire framesynchronization, cell group ID and/or cell CP configuration (i.e.,utilization information of a normal CP or extended CP). Each of PSS andSSS may be transmitted at two OFDM symbols of each radio frame. Inaddition, PSS and SSS are transmitted on 6 RBs (i.e., 3 left RBs and 3right RBs) arranged on the basis of a DC subcarrier within thecorresponding OFDM symbol. PSS is transmitted at the last OFDM symbol(i.e., the 6^(th) OFDM symbol in case of a normal CP, or the 5^(th) OFDMsymbol in case of an extended CP) of a first slot (i.e., Slot 0 or Slot10) of the 0^(th) or 5^(th) subframe within the 10 ms radio frame using72 subcarriers (10 subcarriers are reserved, 62 subcarriers are used forPSS transmission). Although SSS has the same frequency and slotallocation as those of PSS as shown in the drawing. SSS is mappedearlier than a PSS by a predetermined time corresponding to one symbol.

Cell search is performed using the above-mentioned PSS/SSS. The cellsearch may be performed to obtain time and frequency synchronizationwith the cell and a physical layer cell ID (PCI) of the correspondingcell. In a PSS detection procedure, a cell identity within a cellidentity group can be detected. In the SSS detection procedure, the cellidentity group can be detected.

In addition, the 0^(th) subframe may be configured to transmit not onlya synchronization signal but also a PBCH (Physical Broadcast Channel).The content of the PBCH message is denoted by a master information block(MIC) in the RRC layer. In, the PBCH, BCH may include a DL systembandwidth (dl-Bandwidth or DL BW), PHICH configuration, and system framenumber (SFN). Accordingly, the UE receives a PBCH so that it canexplicitly recognize DL BW, SFN, and PHICH configuration information.Meanwhile, the UE can implicitly recognize the number of transmission(Tx) antenna ports of the BS upon receiving a PBCH. Information aboutthe number of transmission antennas of the base station (BS) can beimplicitly signaled because a 16-bit CRC used for PBCH error detectionis masked (e.g., XOR-operated) with a sequence corresponding to thenumber of transmission antennas. Cell-specific scrambling, modulation,layer-mapping, and precoding are performed on a PBCH, so that theprecoded result is mapped to physical resources.

Therefore, in the system, the 0^(th) subframe is configured to transmitSSCH, PSCH and PBCH, and the 5^(th) subframe is configured to transmitSSCH and PSCH. Specifically, in the 0^(th) and 5^(th) subframes, the4^(th) OFDM symbol is an OFDM symbol for SSCH transmission, and the5^(th) OFDM symbol is an OFDM symbol for PSCH transmission. In the0^(th) subframe, 6^(th) to 9^(th) OFDM symbols are used to transmit aPBCH.

When powered on or when entering a new cell, a UE performs initial cellsearch. The initial cell search involves synchronization with a BS.Specifically, the UE synchronizes with the BS and acquires a cellIdentifier (ID) and other information by receiving a PrimarySynchronization Channel (PSS) and a Secondary Synchronization Channel(SSS) from the BS. Then the UE may acquire information broadcast in thecell by receiving a Physical Broadcast CHannel (PBCH) from the BS.

After initial cell search, the UE may acquire more specific systeminformation by receiving a Physical Downlink Control CHannel (PDCCH) andreceiving a Physical Downlink Shared CHannel (PDSCH) based oninformation of the PDCCH. After the above-mentioned procedure, the UEmay receive a PDCCH/PDSCH and transmit a PUSCH)/PUCCH in a generaluplink/downlink signal transmission procedure. Control information thatthe UE transmits to the BS through uplink or another control informationthat the UE receives from the BS may include a DL/UL ACK/NACK signal,Channel Quality Indictor (CQI), a Precoding Matrix Index (PMI), ascheduling request (SR), and a Rank Indicator (RI). CQI, PMI and RI mayalso be referred to as Channel State Information (CSI).

9. Example of Almost Blank Subframe (ABS)

FIG. 17 exemplarily shows an Almost Blank Subframe (ABS) based on theconcept of FIG. 13.

FIG. 17 exemplarily shows influence of DL environment mutualinterference in a heterogeneous network structure. As can be seen fromFIG. 17, the macro UE located at an outer region of the pico BS (or picoeNB) may be seriously interfered by the pico BS. In this case, thecorresponding macro UE is handed over to the pico BS through CREexecution. However, the macro UE is still interfered by the macro BS. Inaddition, the pico UE served by the legacy pico BS is also interfered bythe macro BS, so that it is impossible for the pico UE to detect adesired signal. In conclusion, the best method for enabling the macro BSnot to generate interference in radio resources used by a UE located atan outer region of the pico BS is to empty the corresponding subframe.Based on the above-mentioned concept, ABS technology has been achieved.In accordance with the ABS technology, a specific period in which themacro BS does not transmit data is constructed, the pico BS can allowits own outer UEs to be scheduled into the corresponding subframethrough exchanging the constructed information between base stations oreNBs, so that the occurrence of interference can be prevented.

FIG. 18 shows two kinds of ABS. Referring to FIG. 18, if a normalsubframe for the ABS is used, a Common Reference Signal (CRS) is stilltransmitted. Therefore, influence of CRC interference still remains. Incontrast, in case of multicast/broadcast over a single frequency network(MBSFN), CRC is not transmitted in the data region so that influence ofinterference caused by CRC can be greatly reduced. However, the use ofMBSFM subframe may be limited, so that the ABS pattern is configured andused in consideration of characteristics of a normal subframe and anMBSFN subframe. The corresponding technology can also be applied to anUL environment.

10. Subframe Shift and Symbol Level Shift

FIG. 19 exemplarily shows a subframe shift and a symbol level shift.

Referring to FIG. 19, in case of an ABS used for preventing interferencein the heterogeneous network, a variety of signals (PSS, SSS, PBCH,paging, SIB1 signals) corresponding to a common channel can also be usedto transmit data or information in the corresponding ABS subframe.Therefore, the corresponding signals may generate mutual interference inthe heterogeneous network situation, so that it may be impossible toperform UE access. FIG. 19 shows a symbol-level shift for interferencereduction related to a subframe shift and control region for preventingmutual interference among PSS, SSS and PBCH. As can be seen from FIG.19, if one subframe is shifted, the position among PSS, SSS and PBCHdeviates from the time domain, the mutual corresponding subframe may bedetermined to be an ABS or the mutual corresponding resource is muted sothat interference reduction can be achieved.

Provided that only one subframe shift is used, overlay control regionsmay unavoidably occur, so that interference related to the controlchannel is still serious.

In FIG. 19, if the symbol level shift (assuming that the control regionis composed of 3 OFDM symbols) is performed after completion of thesubframe shift, the corresponding three OFDM symbols may be muted or thecorresponding subframe may be established as the ABS, so thatinterference caused by the control region can be reduced.

FIG. 20 exemplarily shows an ABS, a subframe shift, and a symbol levelshift. In more detail, FIG. 20 shows 2-subframe-shift for reducinginterference of the above-mentioned common channels and a3-OFDM-symbol-shift for protecting the control region, and also shows aframe structure in which a normal subframe and an MBSFN subframe for usein the BS corresponding to an aggressor are allocated to the ABS so thatinterference can be greatly reduced.

11. Scheme for Synchronization

11.1. Inter-Cell Interference Coordination

As can be seen from the above-mentioned heterogeneous network condition,the ABS technology is used to ICIC. In a non-CSG environment based onthe ABS technology, UE measurement related to the corresponding neighborcell must be performed in such a manner that the macro cell located inthe vicinity of the pico cell can be handed over the pico cell or canperform CRE to the pico cell using the UE measurement result. For thispurpose, it is necessary to first detect a synchronization channel of aneighbor cell. However, it may be difficult to correctly detect PSS/SSSfrom the pico cell due to PSS and SSS interference derived from themacro cell. As can be seen from TS36.133, a synchronization-channelrelated requirement (the ratio of the transmit energy per PN chip of theSCH to the total transmit power spectral density) has values of −4 dB˜−6dB according to inter-frequency measurement, intra-frequency measurementor secondary component carrier measurement. Therefore, as can be seenfrom FIG. 11, assuming that the UE configured to perform CRE or handoverdetermines a smaller value than the corresponding request to be a CRE orhandover biasing value, it is impossible to detect the synchronizationchannel of the neighbor cell so that it may be impossible to measure aneighbor cell.

Accordingly, the present invention proposes the following methods toacquire timing synchronization from the neighbor cell as in thecorresponding pico cell.

11.2. Reception of Network Additional Data According to the PresentInvention

If it is impossible to detect PSS/SSS, synchronization can be acquiredusing the CRS of the neighbor cell. However, it is impossible torecognize system information over a BCH of the neighbor cell, so that atleast one of the following additional data is needed whensynchronization is performed using the CRS.

(1) System bandwidth is needed for predicting the number of used RBs andthe number of transmitted CRSs.

(2) Physical cell ID) & v-shift are needed for predicting a CRS sequenceand a CRS pattern.

(3) Configuration of the number of antenna ports is needed forpredicting a CRS pattern.

(4) MBSFN configuration is needed when the ABS of the macro cellperforms synchronization using the pico CRS so as to stably detect thepico-cell CRS.

(5) The number of subframes and the number of slots are needed fordetecting. the correct CRS based on the number of subframes and thenumber of slots on the basis of one reference cell.

(6) Timing different offset is needed for synchronization related totransmission synchronization errors.

(7) “Search window size and/or starting point” are needed for acquiringnot only information about an arbitrary start point calculated by thenetwork but also information about synchronization generated from a CRSpropagation delay within the search window range, when synchronizationis performed using CRC-based correlation.

The legacy OTDOA may refer to the design of additional data (signaled toUE through an LPP) for the neighbor cell, or may perform RRC signalingin the same concept as described above.

11.3. Synchronization Scheme Using CRS

FIG. 21 exemplarily shows autocorrelation based on a common referencesignal (CRS) planted at intervals of three spaces in a frequency domain.

Referring to FIG. 21, when using two antenna ports, CRS is used asspacing (or interval) of 3 subcarriers, so that three peaks occur asshown in FIG. 10 when synchronization is performed in the time domain. Acenter peak point is used as a reference for synchronization. Thus, inorder to solve the ambiguous problem of the remaining peaks, the searchwindow size must be set to about 11.175 us or less (compared to areference point), so that it can prevent at least one peak fromoccurring in a single search window using 66.67 us/3 as a single value.In this case, 66.7 us is a duration of 1 OFDM symbol (other than a CP).

11.4. Obtain Synchronization Using Additional Configured Signal orChannel Other than Legacy PSS/SSS According to the Present Invention

In order to solve interference problem between PSS and SSS in aheterogeneous network condition, the ABS-technology based subframe shiftscheme is proposed as shown in FIG. 20. However, if the MBSFN subframeis configured and used in the FDD, the same subframe synchronization isneeded, so that a method for employing the subframe shift scheme may berestricted. In addition, if different UL/DL configurations areconfigured in the TDD situation, the subframe shift may cause stronginterference between subframes due to UL/DL mismatching. Therefore, thepresent invention proposes an additional signaling method for solvingthe above-mentioned restriction condition and at the same timeperforming synchronization.

In the heterogeneous network (macro-pico) environment based on the ABStechnology 9, the macro UE located in the vicinity of the pico cellsynchronizes with the pico cell acquires synchronization of the picocell, so that it can perform UE measurement using the acquiredsynchronization information. In case of PSS and SSS, if the subframeshift is not present, it is impossible to avoid mutual interference sothat there is difficulty in acquiring synchronization of the pico cell.In this case, the macro BS may plant additional signals forsynchronization acquisition of the corresponding UE into the OFDM symbolhaving no CRS of the corresponding pico subframe of an ABS-configuredsubframe, and may configure the planted result, so that it can obtainsynchronization of the pico cell.

11.4.1. Use of SSS Acting as Additional Signal

In the embodiments of the present invention, SSS is used as anadditional signal in the FDD mode. The present invention can propose whythe SSS but not the PSS is used as additional signals PSS is allocatedearlier than the SSS. The SSS can detect a cell identify (cell ID)group.

FIG. 22 exemplarily shows synchronization acquisition using additionalsignals.

Referring to FIG. 22, it may be difficult to detect PSS and SSS of thepico cell due to interference of the macro cell. As a result, theadditional signal corresponding to the SSS is planted into the pico cellas shown in FIG. 22, so that synchronization acquisition may beachieved. Since strong interference is generated from the macro cell,the corresponding additional signal (SSS) is located at a subframeestablished as ‘ABS’ in the macro BS. If the corresponding pico BS notestablished as ‘ABS’ transmits the SSS in the subframe of the BS, thereis a need for the macro BS corresponding to SSS transmission resourcesto perform muting of the resources. In addition, the ABS has difficultyin avoiding interference from the macro CRS. Accordingly, if the dataregion of a subframe corresponding to either an OFDM symbol having noCRS or an MBSFN-configured ABS is used in the subframe of the pico cellcorresponding to the macro-cell ABS, synchronization acquisition can bestably achieved. For example, if an antenna port is set to 2, SSS can betransmitted without receiving interference from the CRS at some symbols#1, #2, #3, #5, #6, #8, #9, #10, #12 and #13 from among symbols #0-#13,so that synchronization can be efficiently performed. If the antennaport is set to 4, the corresponding symbol number can be sent at symbols#2, #3, #5, #6, #9, #10, #12, #13 without causing CRS interference. Oneor more corresponding additional signals can be transmitted within onesubframe, and associated transmission format may be based on SSS andreuse a new design or legacy signals. If SSS or the like is configuredas shown in FIG. 22, synchronization acquisition errors caused by thelegacy SSS can be distinguished from each other due to fixed symbolpositions of two legacy SSSs. Therefore, assuming that informationexchange or previous acquisition through an X2 interface (or S1interface) between the base stations is achieved, the UE requires thefollowing assistant data (1) and (2).

1) If SSS is used in the embodiment, additional signal information canbe obtained when SSS signal information is provided. In addition, whenacquiring the legacy UE synchronization, an SSS but not a PSS is usedfor solving the ambiguity problem. Legacy common channels may be used asshown in the above-mentioned embodiment, and a new signal may be definedand used.

) Configuration information & location information of Additional Signal:Additional signal configuration may be achieved through higher layersignaling (e.g., RRC signaling). In case of the correspondingconfiguration, the signal position information is fixed to a fixedformat, so that the presence or absence of a signal at the correspondingposition can be indicated using the on/off format of the correspondingsignal. In addition, the semi-static or dynamic-type signal positioninformation can be indicated using a plurality of bits.

11.4.2. Method for Acquiring Synchronization Using InterferenceCancellator

FIG. 23 shows mutual interference environment of a synchronous channelin a heterogeneous network.

Referring to FIG. 23, the UE, that is located at a cell boundary of thelegacy pico cell and receives a service from the macro cell, is unableto directly detect a synchronization channel because the strength of asignal from the pico cell is relatively lower than that of the macrosynchronization channel. However, assuming that there is a largedifference between both signals (i.e., if a handover or CRE biasingvalue is high), after a synchronization channel is acquired from themacro cell, if the corresponding signals are cancelled from thereception signal using the estimated desired channel and the detectedSSs, only synchronization channels of neighbor cells (i.e., pico cells)remain. Therefore, a neighbor synchronization channel can be acquiredusing the non-coherent detection or coherent detection scheme.

(1) The macro UE detects a macro PSS, a pico PSS, and a PSS of areception signal having noise using the non-coherent detection scheme.In this case, a physical layer ID is set to one of three IDs.

(2) The macro UE estimates a channel between the pico BS and the UEusing not only the PSS obtained from the procedure (1) but also channelinformation.

(3) In the simultaneously received SSS channels, both SSSs can bedetected using channel estimation information obtained (or pre-obtained)independently from the procedures (1) and (2).

(3-1) In the method (3), after the SSS is obtained using macro-UEchannel information estimated for macro SSS detection, the macro SSS isremoved from the entire reception signal, and the SSS of the pico BS iscancelled.

(3-2) In the order or method of such cancellation, all cancellationmethods based on the above-mentioned CRE or handover-large biasing valuecan be applied to the present invention.

Besides the above-mentioned methods, even in the case of using a cell IDof a cell adjacent to the network or SS-related sequence information, asynchronization channel can be acquired through direct cancellation.

12. Apparatus Applicable to the Present Invention

FIG. 24 is a block diagram illustrating a wireless communicationapparatus according to an embodiment of the present invention.

Referring to FIG. 24, the wireless communication system includes a basestation (BS) 200 and a plurality of UEs 210 located in the BS region200.

The BS 200 includes a processor 201, a memory 202, and a radio frequency(RF) unit 203. The processor 201 may be constructed to implement theprocedures and/or methods disclosed in the embodiments of the presentinvention. Layers of the radio interface protocol may be implemented bythe processor 201. The memory 202 may be connected to the processor 201,and store various information related to operations of the processor201. The RF unit 203 is connected to the processor 201, and transmitsand/or receives RF signals.

The UE 210 includes a processor 211, a memory 212, and an RF unit 213.The processor 211 may be constructed to implement the procedures and/ormethods disclosed in the embodiments of the present invention. Layers ofthe radio interface protocol may be implemented by the processor 211.The memory 212 may be connected to the processor 211, and store variousinformation related to operations of the processor 211. The RF unit 213is connected to the processor 211, and transmits and/or receives RFsignals.

The memory 202 or 212 may be located inside or outside the processor 201or 211, and may be connected to the processor 201 or 211 through variouswell known means. In addition, the BS 200 and/or the UE 210 may have asingle antenna or multiple antennas.

The aforementioned embodiments are achieved by combination of structuralelements and features of the present invention in a predeterminedfashion. Each of the structural elements or features should beconsidered selectively unless specified otherwise. Each of thestructural elements or features may be carried out without beingcombined with other structural elements or features. Also, somestructural elements and/or features may be combined with one another toconstitute the embodiments of the present invention. The order ofoperations described in the embodiments of the present invention may bechanged. Some structural elements or features of one embodiment may beincluded in another embodiment, or may be replaced with correspondingstructural elements or features of another embodiment. Moreover, it willbe apparent that some claims referring to specific claims may becombined with other claims referring to the other claims other than thespecific claims to constitute the embodiment or add new claims by meansof amendment after the application is filed.

The embodiments according to the present invention can be implemented byvarious means, for example, hardware, firmware, software, orcombinations thereof. If the embodiment according to the presentinvention is implemented by hardware, the embodiment of the presentinvention can be implemented by one or more application specificintegrated circuits (ASICs), digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

If the embodiment according to the present invention is implemented byfirmware or software, the embodiment of the present invention may beimplemented by a module, a procedure, or a function, which performsfunctions or operations as described above. Software code may be storedin a memory unit and then may be driven by a processor. The memory unitmay be located inside or outside the processor to transmit and receivedata to and from the processor through various well known means.

It will be apparent to those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

MODE FOR INVENTION

Various embodiments have been described in the best mode for carryingout the invention.

INDUSTRIAL APPLICABILITY

Although the method for transmitting and receiving data in a wirelessaccess system according to the embodiments of the present invention hasbeen disclosed on the basis of only the 3GPP LTE system application, itshould be noted that the embodiments are also applicable not only to an3GPP LTE system but also to other wireless access systems.

The invention claimed is:
 1. A method for performing synchronizationwith a neighbor base station (BS) of a user equipment (UE) in a wirelessaccess system, the method comprising: when the UE based on a macro BS islocated at a boundary of the neighbor BS, receiving, by the UE, aprimary synchronization signal (PSS) and a secondary synchronizationsignal (SSS) of the neighbor BS interfered by a PSS and a SSS of themacro BS through subframe #0 and subframe #5; receiving, by the UE, anadditional synchronization signal (SS) of the neighbor BS in a subframecorresponding to an almost blank subframe (ABS) of the macro BS wheninformation indicating a presence of the additional SS is received;performing, by the UE, synchronization with the neighbor BS using thePSS and the SSS of the neighbor BS interfered by the PSS and the SSS ofthe macro BS and the additional SS of the neighbor BS, wherein thesubframe corresponding to the ABS of the macro BS is other than thesubframe #0 and the subframe #5, wherein the additional SS is receivedin a fixed position in the subframe corresponding to the ABS of themacro BS, wherein the fixed position is one of symbols #2, #3, #5, #6,#9, #10, #12 and #13 among symbols #0 to #13 in the subframecorresponding to the ABS of the macro BS, and wherein the additional SSof the neighbor BS is different from the PSS or the SSS of the neighborBS.
 2. The method according to claim 1, wherein the subframecorresponding to the ABS of the macro BS has no Common Reference Signal(CRS).
 3. A user equipment (UE) for acquiring synchronization with aneighbor base station (BS) in a wireless access system, the userequipment (UE) comprising: a processor configured to receive a primarysynchronization signal (PSS) and a secondary synchronization signal(SSS) of the neighbor BS interfered by a PSS and a SSS of a macro BSthrough subframe #0 and subframe #5, when the UE based on the macro BSis located at a boundary of the neighbor BS, the processor configured toreceive an additional synchronization signal (SS) of the neighbor BS ina subframe corresponding to an almost blank subframe (ABS) of the macroBS when information indicating a presence of the additional SS isreceived; and the processor configured to perform synchronization withthe neighbor BS using the PSS and the SSS of the neighbor BS interferedby the PSS and the SSS of the macro BS and the additional SS of theneighbor BS, wherein the subframe corresponding to the ABS of the macroBS is other than the subframe #0 and the subframe #5, wherein theadditional SS is received in a fixed position in the subframecorresponding to the ABS of the macro BS, wherein the fixed position isone of symbols #2, #3, #5, #6, #9, #10, #12 and #13 among symbols #0 to#13 in the subframe corresponding to the ABS of the macro BS, andwherein the additional SS of the neighbor BS is different from the PSSor the SSS of the neighbor BS.
 4. The user equipment (UE) according toclaim 3, wherein the subframe corresponding to the ABS of the macro BShas no Common Reference Signal (CRS).